The plasma filtration processes include so-called double filtration plasmapheresis by which the substance to be removed is selected according to the pore size of a secondary filter, and so-called cryofiltration for removing pathogenic substances from plasma by a secondary filter with cooling. In either of these processes, continued filtration of plasma tends to gradually clog the secondary filter, so that it is necessary to eliminate clogging in the course of filtration.
There are two methods of eliminating clogging; one is replacement of the secondary filter by another one, and the other is washing the secondary filter with physiological saline or the like. The latter is used generally since the secondary filter is expensive.
As a method of washing the secondary filter, it is already proposed to wash the filter with a wash liquor in an amount approximately equal to the priming volume of the filter to ten-odd times the volume when the filtration pressure has reached 60% of the withstanding pressure of the secondary filter (maximum pressure to withstand the filtration pressure with safety), for example, 300 mm Hg (e.g., Tetsuzo Agishi et al., "Double Filtration Plasmapheresis," published by Igaku Shoin). However, the conventional washing method which consumes a large amount of wash liquor approximately equal to the priming volume to ten-odd times the volume gives rise to the problem of resulting in a greatly reduced filtration efficiency to necessitate much time for removing the pathogenic substance from plasma because every time the secondary filter is washed, the interior of the filter is entirely replaced by the wash liquor with the cake almost completely washed away by washing. The term filtration efficiency as used herein refers to the rate of removal of the substance per unit time. For example, in the case of cryofiltration wherein usually 2 to 4 liters of plasma needs to be treated in circulation, the time taken is as long as about 4 hours in total including the time required for washing the secondary filter, and the patient must be placed under restraint during this period. It is desired to shorten the time.